The field of the invention is magnetic resonance imaging (“MRI”) methods and systems. More particularly, the invention relates to systems for managing image data associated with MRI.
When a substance such as human tissue is subjected to a uniform magnetic field (polarizing field B0), the individual magnetic moments of the excited nuclei in the tissue attempt to align with this polarizing field, but precess about it in random order at their characteristic Larmor frequency. If the substance, or tissue, is subjected to a magnetic field (excitation field B1) which is in the x-y plane and which is near the Larmor frequency, the net aligned moment, Mz, may be rotated, or “tipped”, into the x-y plane to produce a net transverse magnetic moment Mt. A signal is emitted by the excited nuclei or “spins”, after the excitation signal B1 is terminated, and this signal may be received and processed to form an image.
When utilizing these “MR” signals to produce images, magnetic field gradients (Gx, Gy and Gz) are employed. Typically, the region to be imaged is scanned by a sequence of measurement cycles in which these gradients vary according to the particular localization method being used. The resulting set of received MR signals are digitized and processed to reconstruct the image using one of many well known reconstruction techniques.
The measurement cycle used to acquire each MR signal is performed under the direction of a pulse sequence produced by a pulse sequencer. Clinically available MRI systems, for example, store a library of such pulse sequences that can be prescribed to meet the needs of many different clinical applications.
The MR signals acquired with an MRI system are signal samples of the subject of the examination and are stored as “raw data”, which includes both phase cycling and other data, as well as “k-space data”, which relates the signals to Fourier space, or what is often referred to in the art as “k-space”. Each MR measurement cycle, or pulse sequence, typically samples a portion of k-space along a sampling trajectory characteristic of that pulse sequence. Most pulse sequences sample k-space in a roster scan-like pattern sometimes referred to as a “spin-warp”, a “Fourier”, a “rectilinear” or a “Cartesian” scan. The spin-warp scan technique is discussed in an article entitled “Spin-Warp MR Imaging and Applications to Human Whole-Body Imaging” by W. A. Edelstein et al., Physics in Medicine and Biology, Vol. 25, pp. 751-756 (1980). It employs a variable amplitude phase encoding magnetic field gradient pulse prior to the acquisition of MR spin-echo signals to phase encode spatial information in the direction of this gradient. In a two-dimensional implementation (2DFT), for example, spatial information is encoded in one direction by applying a phase encoding gradient (Gy) along that direction, and then a spin-echo signal is acquired in the presence of a readout magnetic field gradient (Gx) in a direction orthogonal to the phase encoding direction. The readout gradient present during the spin-echo acquisition encodes spatial information in the orthogonal direction. In a typical 2DFT pulse sequence, the magnitude of the phase encoding gradient pulse Gy is incremented (ΔGy) in the sequence of measurement cycles, or “views” that are acquired during the scan to produce a set of k-space MR data from which an entire image can be reconstructed.
There are many other k-space sampling patterns used by MRI systems These include “radial”, or “projection reconstruction” scans in which k-space is sampled as a set of radial sampling trajectories extending from the center of k-space as described, for example, in U.S. Pat. No. 6,954,067. The pulse sequences for a radial scan are characterized by the lack of a phase encoding gradient and the presence of a readout gradient which changes direction from one pulse sequence view to the next. There are also many k-space sampling methods which are closely related to the radial scan and which sample along a curved k-space sampling trajectory rather than the straight line radial trajectory. Such pulse sequences are described, for example, in “Fast Three Dimensional Sodium Imaging”, MRM, 37:706-715, 1997 by F. E. Boada, et al. and in “Rapid 3D PC-MRA Using Spiral Projection Imaging”, Proc. Intl. Soc. Magn. Reson. Med. 13 (2005) by K. V. Koladia et al and “Spiral Projection Imaging: a new fast 3D trajectory”, Proc. Intl. Soc. Mag. Reson. Med. 13 (2005) by J. G. Pipe and Koladia. In projection reconstruction, the number of views needed to sample k-space determines the length of the scan and, if an insufficient number of views are acquired, streak artifacts are produced in the reconstructed image. The technique disclosed in U.S. Pat. No. 6,487,435 reduces such streaking by acquiring successive undersampled images with interleaved views and sharing peripheral k-space data between successive image frames. This method of sharing acquired peripheral k-space data is known in the art by the acronym “TRICKS”.
There are also a number of variations of this straight line, radial sampling trajectory in which a curved path or a non-rectilinear pattern is used to sample k-space. These include spiral projection imaging, SHELL imaging, and the periodically rotated overlapping parallel lines with enhanced reconstruction (PROPELLER) fast spin-echo (FSE) techniques described by Pipe J G, Farthing V G, Forbes K P, “Multishot Diffusion-Weighted FSE Using PROPELLER MRI”, Magn. Reson. Med. 2002; 47:42-52; Forbes K P, Pipe, J G, Karis J P, Heiserman J E, “Improved Image Quality and Detection Of Acute Cerebral Infarction With PROPELLER Diffusion-Weighted MR Imaging”, Radiology 2002; 225:551-555; Forbes K P, Pipe, J G, Karis J P, Farthing V, Heiserman J E, “Brain Imaging In the Unsedated Pediatric Patient: Comparison Of Periodically Rotated Overlapping Parallel Lines With Enhanced Reconstruction and Single-Shot Fast Spin-Echo Sequences”, AJNR Am J Neuroradiol 2003; 24:794-798. This method uses multishot FSE acquisitions incorporated with a k-space trajectory somewhat similar to that used in the projection reconstruction method.
Irrespective of the sampling method used, after k-space data is acquired, images are reconstructed from the acquired k-space data by transforming the k-space data set to an image space data set. There are many different methods for performing this task and the method used is often determined by the technique used to acquire the k-space data. With a Cartesian grid of k-space data that results from a 2D or 3D spin-warp acquisition, for example, the most common reconstruction method used is an inverse Fourier transformation (“2DFT” or “3DFT”) along each of the 2 or 3 axes of the data set. With a radial k-space data set and its variations, the most common reconstruction method includes “re-gridding” or “re-binning” the k-space samples to create a Cartesian grid of k-space samples and then perform a 2DFT or 3DFT on the re-binned k-space data set. In the alternative, a radial k-space data set can also be transformed to Radon space by performing a 1DFT of each radial projection view and then transforming the Radon space data set to image space by performing a filtered back projection.
As discussed above, collected k-space data is a subset of the “raw data”, which is stored in a file called a raw data file in the data storage of an MR imaging system, usually in volatile memory. The format of each raw data file is vendor, and in some cases, scanner-type specific. However, raw data files usually include two parts, a header and a body. The header provides an identifier to the data or gives a file name for the data or provides ancillary non-image information, and typically contains patient information and pulse sequence information, including time of acquisition. The body contains the actual raw data that has been collected during the pulse sequence, usually in a chronological order. Typically, a pulse sequence obtains information from multiple slices of the body. The raw data is not slice specific and contains data from all slices selected prior to running the pulse sequence. The raw data in the data section is often maintained or stored in volatile memory in a sequence that is in the order of data acquisition.
After image reconstruction, the reconstructed image is stored in an MRI image file, which can be stored either locally, or in a Picture Archive Communication System (PACS). MR image files are usually in a vendor-independent format called DICOM. Using the DICOM format, each MR image file has a header portion and a body portion. The header portion contains information similar to that located in the raw data header as well as information about the specific corresponding imaging slice, e.g. image slice number. The body portion contains the actual image data. Typically, each MR image file contains image data about one imaging slice.
Although the MR image file is retained, the “raw files” including the underlying k-space data are not retained, or at best are retained only in the vendor-specific format. Therefore, in these prior art methods, the underlying data acquired during the scan is lost, and if additional views or visualization is required beyond the reconstructed scans, additional scans must be performed, and new sets of data must be acquired. It would be desirable, therefore, to retain a representation of the underlying k-space data and the associated identifying data to allow for additional images to produced after the data is acquired.